Ignition plug and method for manufacturing ignition plug

ABSTRACT

An ignition plug such as a spark plug or a glow plug configured to ignite an air-fuel mixture in an internal combustion engine includes a mark that is formed of an oxide film generated on the surface of a metal member or is formed of the metal member and the oxide film. The mark is formed by promoting formation of the oxide film on the surface of the metal member or removing the oxide film through radiation of a laser beam onto the surface of the metal member.

RELATED APPLICATIONS

This application claims the benefit of Japanese Patent Application No.2017-094516, filed May 11, 2017, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an ignition plug and a method formanufacturing the ignition plug, and more particularly, to an ignitionplug having a mark and a method for manufacturing the ignition plug.

BACKGROUND OF THE INVENTION

There is a technology in which a predefined identifier (mark) isprovided on an industrial product and background information on theindustrial product becomes traceable throughout procurement, processing,production, distribution, sales, disposal, and the like. Ignition plugsconfigured to ignite air-fuel mixtures in internal combustion enginesare also required to have marks in order to increase the traceability.As a technology for providing a mark on an industrial product, JapaneseRegistered Utility Model No. 3078913 discloses a technology of printinga mark by radiating a laser beam onto the surface of a ceramic base.

In the related art, however, the mark is formed by radiating the laserbeam onto the surface of the ceramic base that is a brittle material.Therefore, the mark may trigger breaking of the base, thereby causing arisk that the strength of the member having the mark may decrease.

SUMMARY OF THE INVENTION

The present invention has been made to address the problem describedabove. An advantage of the present invention is an ignition plug and amethod for manufacturing the ignition plug, which are capable ofsuppressing a decrease in the strength of a member having a mark.

In order to achieve this advantage, there is provided an ignition plugaccording to the present invention is configured to ignite an air-fuelmixture in an internal combustion engine. The ignition plug includes amark that is formed of an oxide film generated on a surface of a metalmember or is formed of the metal member and the oxide film.

According to the present invention, there is provided a method formanufacturing an ignition plug configured to ignite an air-fuel mixturein an internal combustion engine. The method includes a preparation stepof preparing a metal member that constitutes a part of the ignitionplug, and a marking step of forming a mark that is formed of an oxidefilm or is formed of the metal member and the oxide film by radiating alaser beam onto a surface of the metal member.

According to a first aspect, in the ignition plug, the mark is firmed ofthe oxide film generated on the surface of the metal member or is formedof the metal member and the oxide film. Thus, the decrease in thestrength of the member having the mark can be suppressed compared with acase in which the mark is provided on a ceramic member.

According to a second aspect, in the ignition plug, the mark includes afirst portion, and a second portion having a reflectance higher than areflectance of the first portion. A variation of distribution of thereflectance of the first portion or a variation of distribution of thereflectance of the second portion is smaller than a variation ofdistribution of a reflectance of a portion on the surface of the metalmember adjacent to a periphery of the mark or a variation ofdistribution of a reflectance of an oxide film adjacent to a peripheryof the mark.

Therefore, when the mark is read by detecting beams of reflected light,a threshold can be set by using the distribution of the reflectance.Thus, in addition to the advantage of the first aspect, the mark readingaccuracy relative to the portion adjacent to the periphery of the markcan be improved.

According to a third aspect, in the ignition plug, the variation of thedistribution of the reflectance of the first portion d the variation ofthe distribution of the reflectance of the second portion are smallerthan the variation of the distribution of the reflectance of the portionon the surface of the metal member adjacent to the periphery of the markor a variation of distribution of a reflectance of an oxide filmadjacent to a periphery of the mark.

Thus, in addition to the advantage of the second aspect, the markreading accuracy can further be improved.

According to a fourth aspect, the ignition plug further includes aninsulator, a tubular metal shell that holds the insulator inside, aground electrode joined to the metal shell, and a tip that is joined tothe ground electrode and contains a noble metal. The ground electrodeincludes a non-plated portion having no plating film formed on a surfacethereof. The mark is provided on the non-plated portion of the groundelectrode. Thus, in addition to the advantage of any one of the first tothird aspects, the mark provided on the ground electrode can beprevented from being lost even when processing for removing the platingfilm formed on the ground electrode is performed.

According to a fifth aspect, the ignition plug further includes aninsulator extending in a direction of an axial line, and a metalterminal fixed to an end of the insulator in the direction of the axialline. The mark is provided on an end face of the metal terminal in thedirection of the axial line. The mark can be read in the direction ofthe axial line of the ignition plug. Thus, in addition to the advantageof any one of the first to fourth aspects, the mark can be read moreeasily than in a case in which the mark is provided on the side face ofthe ignition plug.

According to a sixth aspect, in the ignition plug, the metal terminalincludes a wall portion that surrounds the end face. Thus, in additionto the advantage of the fifth aspect, the mark can be protected by thewall portion so that the mark is not damaged or rubbed.

According to a seventh aspect, in the method for manufacturing anignition plug, the metal member that constitutes a part of the ignitionplug is prepared in the preparation step. In the marking step, the markthat is formed of the oxide film or is formed of the metal member andthe oxide film is formed by radiating the laser beam onto the surface ofthe metal member. Thus, the decrease in the strength of the metal memberhaving the mark can be suppressed.

According to an eighth aspect, in the method for manufacturing anignition plug, the mark includes a first portion, and a second portionhaving a reflectance higher than a reflectance of the first portion. Ina first step of the marking step, the laser beam is radiated onto aportion on the surface of the metal member where the first portion is tobe formed or a portion on the surface of the metal member where thesecond portion is to be formed. Thus, in addition to the advantage ofthe seventh aspect, a mark to be read by detecting beams of reflectedlight can be formed.

According to a ninth aspect, in the method for manufacturing an ignitionplug, in a second step of the marking step, after the first step, thelaser beam is radiated onto the portion where the first portion is to beformed but the laser beam is not radiated in the first step or theportion where the second portion is to be formed but the laser beam isnot radiated in the first step. As a result, the contrast between thefirst portion and the second portion can be enhanced. Thus, in additionto the advantage of the eighth aspect, the mark reading accuracy can beimproved.

According to a tenth aspect, in the method for manufacturing an ignitionplug, in an underlayer forming step of the marking step, before thefirst step, an underlayer region is formed by radiating the laser beamonto a portion where the mark is to be formed. Since the mark is formedon the underlayer region, the contrast between the mark and the portionadjacent to the periphery of the underlayer region can be enhanced.Thus, in addition to the advantage of the eighth or ninth aspect, themark reading accuracy can further be improved.

According to an eleventh aspect, in the method for manufacturing anignition plug, the first portion is formed by promoting formation of theoxide film through radiation of the laser beam. The second portion isformed by removing the oxide film through radiation of the laser beam.Thus, in addition to the advantage of any one of the eighth to tenthaspects, the first portion and the second portion can be formed bycontrolling a laser output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a half sectional view of an ignition plug according to oneembodiment of the present invention.

FIG. 2 is a rear view of the ignition plug that is viewed in a directionof the arrow II in FIG. 1.

FIG. 3A is a sectional view of a metal terminal taken along an axialline. underlayer region is formed by radiating a laser beam.

FIG. 3C is a sectional view of the metal terminal taken along the axialline, on which a first portion of a mark is formed by radiating thelaser beam.

FIG. 3D is a sectional view of the metal terminal taken along the axialline, on which a second portion of the mark is formed by radiating thelaser beam.

FIG. 4A is a schematic view of beams of reflected light from the mark.

FIG. 4B is a schematic diagram of distribution of reflectance detectedby a light receiving element that receives the beams of reflected lightfrom the mark.

FIG. 5 is a side view of the ignition plug that is viewed in a directionof the arrow V in FIG. 1.

FIG. 6A is a sectional view of a ground electrode on which a mark isformed and a metal shell.

FIG. 6B is a sectional view of the ground electrode on which a platingfilm is formed.

FIG. 7 is a sectional view of the ground electrode to which a tip iswelded and the metal shell.

FIG. 8A is a sectional view of the metal shell on which a mark is formedby radiating the laser beam.

FIG. 8B is a schematic view of beams of reflected light from the mark.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention is described below withreference to the accompanying drawings. FIG. 1 is a half sectional viewof an ignition plug 10 according to one embodiment of the presentinvention. In FIG. 1, the lower side of the drawing sheet is referred toas a front end side of the ignition plug 10 and the upper side of thedrawing sheet is referred to as a rear end side of the ignition plug 10.The ignition plug 10 is configured to ignite an air-fuel mixture in aninternal combustion engine (not illustrated). The ignition plug 10includes an insulator 11, a metal terminal 20, a metal shell 30, and aground electrode 31.

The insulator 11 is a cylindrical member formed of, for example, aluminathat is excellent in mechanical properties and insulation propertiesunder high temperature. An axial hole 12 is formed through the insulator11 along an axial line O. A center electrode 14 is arranged on the frontend side of the axial hole 12.

The center electrode 14 is a rod-shaped member extending along the axialline O. A core material that is copper or contains copper as a maincomponent is covered with nickel or a nickel-based alloy. The centerelectrode 14 is held by the insulator 11 and the front end is exposedfrom the axial hole 12. A tip 15 containing a noble metal is joined tothe front end of the center electrode 14.

The metal terminal 20 is a rod-shaped member to which a high-voltagecable (not illustrated) is connected. The metal terminal 20 is formed ofa conductive metal material (for example, low-carbon steel). In themetal terminal 20, a shaft portion 21 inserted into the axial hole 12, afixing portion 22 fitted to the axial hole 12, and a head portion 23that abuts against a rear end face 13 of the insulator 11 arecontinuously connected to each other. The head portion 23 is formed intoa disc shape and the outer diameter of the head portion 23 is largerthan the outer diameter of the shaft portion 21. The head portion 23 isprovided with a cylindrical wall portion 25 that surrounds an end face24 of the metal terminal 20 in a direction of the axial line O.

In this embodiment, the metal terminal 20 is subjected to nickel platingin order to improve the corrosion resistance of the metal terminal 20. Aheight T of the wall portion 25 from the end face 24 in the direction ofthe axial line O is set to, for example, 2 mm or less.

The metal shell 30 is a substantially cylindrical member formed of aconductive metal material (for example, low-carbon steel). The metalshell 30 is fixed by crimping to the front end side of the outerperiphery of the insulator 11 with a distance from the head portion 23of the metal terminal 20 in the direction of the axial line O. In thisembodiment, a plating film (described later) is formed on the metalshell 30 in order to improve the corrosion resistance of the metal shell30. The ground electrode 31 is joined to the front end of the metalshell 30.

The ground electrode 31 is a rod-shaped metal (for example, nickel-basedmember including a first portion 32 located on the front end side, and asecond portion 33 located on a back side of the first portion 32. Inthis embodiment, a plating film is formed on the ground electrode 31 andis then stripped before a tip 35 (described later) is welded. Thus, anon-plated portion 34 having no plating film is provided on the groundelectrode 31 at least on the same surface as that of the second portion33 and at a root on the rear end side.

The tip 35 containing a noble metal is joined to the first portion 32 ofthe ground electrode 31. In this embodiment, the ground electrode 31 isbent on the front end side and the first portion 32 faces the centerelectrode 14 (tip 15). The tip 35 joined to the first portion 32 forms aspark gap between the tip 35 and the center electrode 14 (tip 15).

For example, the ignition plug 10 is manufactured by the followingmethod. First, the center electrode 14 having the tip 15 joined to thefront end in advance is inserted into the axial hole 12 of the insulator11 and is arranged so that the front end of the center electrode 14 isexposed to the outside from the axial hole 12. The shaft portion 21 ofthe metal terminal 20 is inserted into the axial hole 12 and continuityis secured between the metal terminal 20 and the center electrode 14.Then, the metal shell 30 having the ground electrode 31 joined inadvance is attached to the outer periphery of the insulator 11. The tip35 is joined to the ground electrode 31 and then the ground electrode 31is bent so that the tip 35 faces the center electrode 14 (tip 15). Thus,the ignition plug 10 is obtained.

FIG. 2 is a rear view of the ignition plug 10 that is viewed in adirection of the arrow II in FIG. 1. The metal terminal 20 is oxidizedby being affected by a thermal history involving heat treatmentperformed when the continuity is secured between the metal terminal 20and the center electrode 14 and baking treatment performed when platingis performed. An oxide film 26 obtained by oxidizing the surface of thehead portion 23 is formed on the end face 24 of the metal terminal 20(see FIG. 1). A mark 40 is formed at the center of the end face 24 ofthe metal terminal 20 where the oxide film 26 is formed. In thisembodiment, the mark 40 is a two-dimensional code. Examples of thetwo-dimensional code include PDF417, Micro PDF417, CODE49, Maxicode,Data Matrix, QR code, and Aztec. A one-dimensional code may be formed onthe metal terminal 20.

The end face 24 of the metal terminal 20 where the mark 40 is formed issurrounded by the wall portion 25. Therefore, the mark 40 can beprotected by the wall portion 25. Thus, damage to or rubbing of the mark40 can be prevented.

The mark 40 includes a first portion 41 that is a group of rectangularcells, and a second portion 42 that is a group of rectangular cellshaving a reflectance higher than that of the first portion 41. In thisembodiment, the first portion 41 is a dark module and the second portion42 is a light module. A combination of the first portion 41 and thesecond portion 42 represents background information unique to a productor component. A margin (quiet zone) 43 that corresponds to the edges ofthe mark 40 and demarcates the mark 40 from a portion adjacent to theperiphery of the mark 40 (oxide film 26) is a part of the second portion42 having a reflectance higher than that of the first portion 41.

The edges of the margin 43 of the mark 40 are formed at positions wherethe shortest distance from the wall portion 25 is a predeterminedshortest distance D. This is because the mark 40 can be read by usingreflected light without difficulty that may be caused by blocking of thereflected light or illumination light due to the wall portion 25.

A marking step of forming the mark 40 on the end face 24 of the metalterminal 20 is described with reference to FIG. 3A to FIG. 3D. FIG. 3Ais a sectional view of the metal terminal 20 (head portion 23). FIG. 3Bis a sectional view of the metal terminal 20 on which an underlayerregion 44 is formed by radiating a laser beam 51. FIG. 3C is a sectionalview of the metal terminal 20 on which the first portion 41 of the mark40 is formed by radiating the laser beam 51, FIG. 3D is a sectional viewof the metal terminal 20 on which the second portion 42 of the mark 40is formed by radiating the laser beam 51.

As illustrated in FIG. 3A, the oxide film 26 obtained by oxidizing thesurface of a nickel plating film (not illustrated) is formed on the headportion 23 (metal terminal 20). The oxide film 26 has unevenness in thethickness and density. Therefore, the oxide film 26 is recognized byunaided eyes as being gray or dark gray in a color density depending onthe thickness and density.

FIG. 3B illustrates an underlayer forming step of forming the underlayerregion 44. In the underlayer forming step, the oxide film 26 is removedby irradiating the head portion 23 with the laser beam 51 emitted from aprocessing head 50. At a portion where the mark 40 (see FIG. 2) is to beformed, the rectangular underlayer region 44 (background) is formed byrelatively moving the processing head 50 along the end face 24 of thehead portion 23 (see FIG. 1) and scanning the end face 24 with the laserbeam 51.

When the underlayer region 44 is formed, necessary and sufficient energyto remove the oxide film 26 is input to the head portion 23 by adjustinga laser output from the processing head 50, a scanning speed, a focusdiameter and a focal depth of the laser beam 51, and the like. This isbecause new oxidation of the portion irradiated with the laser beam 51is suppressed to the extent possible while removing the oxide film 26.As a result, the reflectance can be increased while reducing a variationin the color density of the background of the mark 40 (underlayer region44).

FIG. 3C illustrates a first step of forming the first portion 41. In thefirst step, the underlayer region 44 is partially heated by irradiatingthe underlayer region 44 with the laser beam 51 emitted from theprocessing head 50. This operation promotes formation of an oxide filmat the portion heated by radiating the laser beam 51. The first portion41 is formed by relatively moving the processing head 50 along the endface 24 of the head portion 23 (see FIG. 1) and scanning the end face 24with the laser beam 51.

When the first portion 41 is formed, energy having a higher level thanthat for forming the underlayer region 44 is input to the head portion23 by adjusting the laser output from the processing head 50, thescanning speed, the focus diameter and the focal depth of the laser beam51, and the like. This operation promotes oxidation of the portionirradiated with the laser beam 51, thereby forming an oxide film on thefirst portion 41 so that the first portion 41 is turned black.

The degree of oxidation of the first portion 41 can be controlled by thelaser output. Therefore, the thickness and density of the oxide film ofthe first portion 41 can be made substantially uniform. Thus, thecontrast of the first portion 41 can be enhanced compared with the oxidefilm 26 around the mark 40.

The first portion 41 is formed on the underlayer region 44 having asmall variation in the color density. Therefore, unevenness of the firstportion 41 can be eliminated and the contrast of the first portion 41can be enhanced compared with a case in which the first portion 41 isformed on the oxide film 26 by radiating the laser beam 51 withoutforming e underlayer region 44. This is because the first portion 41formed on the oxide film 26 having a variation in the color density byradiating the laser beam 51 reflects the condition of the oxide film 26and therefore the variation in the color density also appears in thefirst portion 41.

FIG. 3D illustrates a second step of forming the second portion 42. Inthe second step, an oxide film generated at a contour of the secondportion 42 by being thermally affected during the formation of the firstportion 41 is removed by radiating the laser beam 51 emitted from theprocessing head 50 onto a portion that is not irradiated with the laserbeam 51 in the first step. The second portion 42 is formed by relativelymoving the processing head 50 along the end face 24 of the head portion23 (see FIG. 1) and scanning the end face 24 with the laser beam 51.Thus, the boundary between the first portion 41 and the second portion42 is clearly defined.

When the second portion 42 is formed, energy having a substantiallyequal level to that for forming the underlayer region 44 is input to thehead portion 23 by adjusting the laser output from the processing head50, the scanning speed, the focus diameter and the focal depth of thelaser beam 51, and the like. By radiating the laser beam 51 also ontothe portion other than the boundary between the first portion 41 and thesecond portion 42, the second portion 42 can be turned white and foulingon the second portion 42 that is generated, for example, during theformation of the first portion 41 can be removed. Thus, the contrastbetween the first portion 41 and the second portion 42 can be enhancedwhile improving the dimensional accuracies of the first portion 41 andthe second portion 42.

Next, beams of reflected light from the mark 40 are described withreference to FIG. 4A and FIG. 4B. FIG. 4A is a schematic view of thebeams of reflected light from the mark 40. The mark 40 is read in such amanner that illumination light 54 is radiated onto the mark 40 and alight receiving element 53 detects beams of reflected light 55, 56, and57 from the mark 40. The light receiving element 53 is a part of animage capturing element such as a CCD or a CMOS, which is provided witha condenser lens and a color filter. The first portion 41 absorbs moreillumination light 54 than the second portion 42 does. Therefore, thelight receiving element 53 can receive the reflected light 56 from thesecond portion 42 more than the reflected light 55 from the firstportion 41.

FIG. 4B is a schematic diagram of distribution of reflectance detectedby the light receiving element 53 that receives the beams of reflectedlight 55 and 56 from the mark 40. FIG. 4B schematically illustratesdistribution of reflectance values indicating the intensities of lightreceived by individual pixels of the light receiving element. 53. InFIG. 4B, the horizontal axis represents the reflectance and the verticalaxis represents the number of pixels (frequency) of the light receivingelement 53 that has detected light.

As illustrated in FIG. 4B, a variation of the distribution of thereflectance of the reflected light 55 from the first portion 41 can bekept smaller than a variation of the distribution of the reflectance ofthe reflected light 57 from the oxide film 26 (around the mark 40).Further, a variation of the distribution of the reflectance of thereflected light 56 from the second portion 42 can be kept smaller thanthe variation of the distribution of the reflectance of the reflectedlight 57 from the oxide film 26 (around the mark 40). This is becausethe degrees of oxidation of the surfaces of the first portion 41 and thesecond portion 42 can be controlled by adjusting the laser output andthe like.

The reflectance of the reflected light 56 from the second portion 42 ishigher than the reflectance of the reflected light 55 from the firstportion 41. Therefore, a reflectance value between the distribution ofthe reflected light 56 and the distribution of the reflected light 55 isset as a threshold. Thus, the first portion 41 (dark module) and thesecond portion 42 (light module) can be read with higher accuracy thanin a case in which the oxide film 26 is used as the dark module. As aresult, the reading rate of the light receiving element 53 can besecured and erroneous reading can be prevented.

In the ignition plug 10, the first portion 41 of the mark 40 is fannedof the oxide film generated on the surface of the metal terminal 20 andthe second portion 42 is formed of the metal terminal 20. Thus, adecrease in the strength of the ignition plug 10 can be suppressedcompared with a case in which the mark 40 is provided on a ceramicmember such as the insulator 11 that is a brittle material.

A thin oxide film may be formed on the surface of the metal terminal 20by adjusting the laser output and the like for forming the secondportion 42 and the thin oxide film may be used as the second portion 42.This is because the mark 40 that can be used only needs to secure thecontrast between the first portion 41 and the second portion 42.

The mark 40 is provided on the end face 24 of the metal terminal 20 andcan therefore be read in the direction of the axial line O of theignition plug 10 even if, for example, the ignition plugs 10 are bundledtogether. Thus, the mark 40 can be read more easily than in a case inwhich the mark 40 is provided on the side face of the ignition plug 10,such as the side face of the metal shell 30 or the ground electrode 31.

FIG. 5 is a side view of the ignition plug 10 (part of the groundelectrode 31) that is viewed in a direction of the arrow V in FIG. 1. InFIG. 5, illustration of both sides of the metal shell 30 and the groundelectrode 31 in the direction of the axial line O is omitted. Asillustrated in FIG. 5, a mark 60 is formed on the non-plated portion 34of the ground electrode 31. In this embodiment, the mark 60 is aone-dimensional code (barcode). Examples of the one-dimensional codeinclude CODE39, CODE93, CODE128, UTA/A, UPA/E, NW-7, GS1-128, JAN8, andJAN13. A two-dimensional code may be formed on the ground electrode 31.

The mark 60 includes a first portion 61 that is a group of rectangularbars, and a second portion 62 that is a group of rectangular bars havinga reflectance higher than that of the first portion 61. In thisembodiment, the first portion 61 is a dark module and the second portion62 is a light module. A combination of the first portion 61 and thesecond portion 62 represents background information unique to a productor component. A margin (quiet zone) 63 that corresponds to both ends ofthe mark 60 and demarcates the mark 60 from portions adjacent to bothends of the mark 60 is a part of the second portion 62.

Next, a marking step of forming the mark 60 on the ground electrode 31is described with reference to FIG. 6A to FIG. 7. FIG. 6A is a sectionalview of the ground electrode 31 on which the mark 60 is formed and themetal shell 30. FIG. 6B is a sectional view of the ground electrode 31on which a plating film 64 is formed. FIG. 7 is a sectional view of theground electrode 31 to which the tip 35 is welded and the metal shell30. FIG. 6A and FIG. 7 are sectional views including the axial line O,illustrating the metal shell 30 and the ground electrode 31. FIG. 6A andFIG. 7 illustrate a state before the metal shell 30 is attached to theinsulator 11 and before the ground electrode 31 is bent.

As illustrated in FIG. 6A, the mark 60 (see FIG. 5) is formed byradiating the laser beam 51 emitted from the processing head 50 onto thenon-plated portion 34 of the ground electrode 31 joined to the metalshell 30. In a first step, the non-plated portion 34 is partially heatedby radiating the laser beam 51 onto the non-plated portion 34. Thisoperation promotes formation of an oxide film at the portion irradiatedwith the laser beam 51. The first portion 61 is formed by relativelymoving the processing head 50 along the non-plated portion 34 andscanning the non-plated portion 34 with the laser beam 51. The degree ofoxidation of the first portion 61 can be controlled by the laser output.Therefore, the thickness and density of the oxide film of the firstportion 61 can be made substantially uniform. Thus, the contrast of thefirst portion 61 can be enhanced compared with a bare surface of thenon-plated portion 34 around the mark 60.

In a second step, an oxide film generated at a contour of the secondportion 62 by being thermally affected during the formation of the firstportion 61 is removed by radiating the laser beam 51 onto a portion thatis not irradiated with the laser beam 51 in the first step. The secondportion 62 is formed by relatively moving the processing head 50 alongthe non-plated portion 34 and scanning the non-plated portion 34 withthe laser beam 51.

When the second portion 62 is formed, energy having a lower level thanthat for fanning the first portion 61 is input to the non-plated portion34 by adjusting the laser output, the scanning speed, the focus diameterand the focal depth of the laser beam 51, and the like. As a result, thesecond portion 62 can be turned white and the contrast of the firstportion 61 to the bare surface of the non-plated portion 34 and thesecond portion 62 can be enhanced. The mark 60 represents backgroundinformation on procurement and processing of the metal shell 30 and theground electrode 31.

As illustrated in FIG. 6B, the plating film 64 is formed on the surfacesof the ground electrode 31 and the metal shell 30 (see FIG. 6A). Theplating film 64 is a surface treatment layer for mainly improving thecorrosion resistance of the metal shell 30. For example, the platingfilm 64 mainly contains zinc, zinc subjected to chromate treatment, ornickel. The plating film 64 is formed by performing barrel platingtreatment on the metal shell 30 to which the ground electrode 31 isjoined. As a result, the plating film 64 is formed not only on thesurface of the metal shell 30 but also on the surface of the groundelectrode 31. Thus, the first portion 61 and the second portion 62 (mark60) are also covered with the plating film 64.

As illustrated in FIG. 7, the plating film 64 formed on the groundelectrode 31 is removed before the tip 35 is welded. This is becausepoor welding due to the plating film 64 is prevented. The plating film64 is partially removed by physical removing means such as ion etchingor shot blasting or by chemical removing means for immersing the groundelectrode 31 in a stripping solution. When the plating film 64 isremoved by the physical or chemical removing means, the mark 60 that hasbeen covered with the plating film 64 appears.

As described above, the mark 60 is formed on the non-plated portion 34of the ground electrode 31. Therefore, the mark 60 provided on theground electrode 31 can be prevented from being lost even when theprocessing for removing the plating film 64 formed on the groundelectrode 31 is performed. The mark 60 is formed on the same surface asthat of the second portion 33 located on the back side of the firstportion 32 to which the tip 35 is joined and at the root on the rear endside of the ground electrode 31. Therefore, the mark 60 can be preventedfrom being burnt off due to spark discharge caused between the groundelectrode 31 and the center electrode 14.

Next, a modified example of a mark 70 is described with reference toFIG. 8A and FIG. 8B. In the mark 70 of the modified example, a secondportion 72 is formed by using a metallic luster of the metal shell 30.FIG. 8A is a sectional view of the metal shell 30 on which the mark 70is formed by radiating the laser beam 51.

As illustrated in FIG. 8A, in a marking step, the laser beam 51 isradiated onto the surface of the metal shell 30 and the portionirradiated with the laser beam 51 is oxidized. A first portion 71 isformed of an oxide film on the surface of the metal shell 30 byrelatively moving the processing head 50 along the surface of the metalshell 30 and scanning the surface with the laser beam 51. The secondportion 72 is a portion that is not irradiated with the laser beam 51for forming the first portion 71. Thus, the mark 70 (one-dimensionalcode or two-dimensional code) is formed so as to have the first portion71 that is recognized by unaided eyes as being black, and the secondportion 72 whose metallic luster is recognized by unaided eyes.

FIG. 8B is a schematic view of beams of reflected light from the mark70. The mark 70 is read in such a manner that illumination light 73 isradiated onto the mark 70 and the light receiving element 53 detectsbeams of reflected light 74 and 75 from the mark 70. The first portion71 and the second portion 72 diffusely reflect the illumination light73. The first portion 71 formed of the oxide film absorbs moreillumination light 73 than the second portion 72 does. Therefore, theintensity of the reflected light 75 from the first portion 71 can bekept lower than the intensity of the reflected light 74 from the secondportion 72. As a result, the light receiving element 53 can detect themark 70 with the first portion 71 serving as a dark module and thesecond portion 72 serving as a light module.

Considering reflectance detected by the light receiving element 53 thatreceives the beams of reflected light 74 and 75 from the mark 70, avariation of distribution of the reflectance of the reflected light 75from the first portion 71 formed by radiating the laser beam 51 can bekept smaller than a variation of distribution of the reflectance of thereflected light 74 from a portion around the mark 70 (the same textureas that of the second portion 72). This is because the degree ofoxidation of the first portion 71 can be controlled by the laser outputand the like. The reflectance value of the reflected light 7.4 detectedby the light receiving element 53 is higher than the reflectance valueof the reflected light 75 detected by the light receiving element 53.Therefore, a reflectance value between the distribution of the reflectedlight 74 and the distribution of the reflected light 75 is set as athreshold. Thus, the first portion 71 (dark module) and the secondportion 72 (light module) can be read with high accuracy.

In the marking step of forming the mark 70, the step of radiating thelaser beam 51 onto the metal shell 30 in order to form the secondportion 72 can be omitted. Thus, the period of time required for themarking step can be shortened.

The present invention has been described above based on the embodimentbut is not limited to the embodiment described above. It can easily beunderstood that various modifications may be made without departing fromthe spirit of the present invention.

The embodiment described above is directed to the ignition plug 10configured to ignite an air-fuel mixture by causing spark dischargebetween the center electrode 14 and the ground electrode 31 (sparkplug). The present invention is not necessarily limited to this case.The mark may be formed on an ignition plug configured to ignite anair-fuel mixture by causing barrier discharge or arc discharge whileomitting the ground electrode 31. Further, the mark may be formed on anignition plug including a heater (glow plug).

The embodiment described above is directed to the case in which themarks are formed on the metal terminal 20, the metal shell 30, and theground electrode 31 of the ignition plug 10. The present invention isnot necessarily limited to this case. The mark may be formed on a metalmember (for example, the center electrode 14) other than the metalterminal 20, the metal shell 30, and the ground electrode 31 e Thecenter electrode 14 has a portion that cannot be viewed from theoutside. The mark that represents background information is provided onthe center electrode 14 and the center electrode 14 is pulled out bydisassembling the ignition plug 10. In this manner, the mark can be readso as to refer to the background information.

The embodiment described above is directed to the case in which theunderlayer region 44 is formed and then the first portion 41 and thesecond portion 42 are formed during the formation of the mark 40. Thepresent invention is not necessarily limited to this case. For example,the second step of forming the second portion 42 may be omitted. Sincethe underlayer region 44 is formed, a portion of the underlayer region44 other than the first portion 41 may be used as the second portion 42even if the second step is omitted as long as the first portion 41 isformed on the underlayer region 44.

When the lightness of the oxide film 26 is high (the reflectance ishigh), the underlayer forming step of forming the underlayer region 44may be omitted and the first portion 41 may be formed on the oxide film26 by radiating the laser beam 51 onto the oxide film 26 in the firststep during the formation of the mark 40. Since the lightness of theoxide film 26 is high, a portion of the oxide film 26 other than thefirst portion 41 may be used as the second portion 42 as long as thefirst portion 41 is formed on the oxide film 26. In this case, the firstportion 41 and the second portion 42 (entire mark 40) are formed of theoxide film.

After the first portion 41 is fanned, the oxide film 26 may be removedto form the second portion 42 by radiating the laser beam 51 onto aportion other than the first portion 41 in the second step. In thiscase, the first portion 41 of the mark 40 is formed of the oxide film.When the oxide film 26 is left as a thin film at, the position of thesecond portion 42 by reducing the focal depth of the laser beam 51 forforming the second portion 42, the entire mark 40 is formed of the oxidefilm.

When the lightness of the oxide film 26 is low (the reflectance is low),the underlayer forming step of forming the underlayer region 44 may beomitted and a part of the oxide film 26 may be removed to form thesecond portion 42 by radiating the laser beam 51 onto the oxide film 26in the first step during the formation of the mark 40. Since thelightness of the oxide film 26 is low, a portion of the oxide film 26other than the second portion 42 may be used as the first portion 41 aslong as the second portion 42 is formed on the oxide film 26. In thiscase, the first portion 41 of the mark 40 is formed of the oxide film26. When the oxide film 26 is left as a thin film at the position of thesecond portion 42 by reducing the focal depth of the laser beam 51 forforming the second portion 42, the entire mark 40 is formed of the oxidefilm 26.

After the second portion 42 is formed, oxidation may be promoted to formthe first portion 41 by radiating the laser beam 51 onto a portion otherthan the second portion 42 in the second step. In this case, the firstportion 41 of the mark 40 is formed of the oxide film. When the oxidefilm 26 is left as a thin film at the position of the second portion 42by reducing the focal depth of the laser beam 51 for forming the secondportion 42, the entire mark 40 is formed of the oxide film.

The embodiment described above is directed to the case in which thefirst portion 61 is formed in the first step and the second portion 62is formed in the second step during the formation of the mark 60. Thepresent invention is not necessarily limited to this case. When thelightness of the non-plated portion 34 is high (the reflectance ishigh), the second step may be omitted. Since the lightness of thenon-plated portion 34 is high, a portion of the non-plated portion 34other than the first portion 61 may be used as the second portion 62 aslong as the first portion 61 is formed on the non-plated portion 34. Theunderlayer region 44 may be formed before the first step.

Also when the lightness of the non-plated portion 34 is low (thereflectance is low), the second step may be omitted during the formationof the mark 60. Since the lightness of the non-plated portion 34 is low,a portion of the non-plated portion 34 other than the second portion 62may be used as the first portion 61 as long as the second portion 62 isformed on the non-plated portion 34.

The embodiment described above is directed to the case in which the mark60 is formed before the plating film 64 is formed on the metal shell 30and the ground electrode 31. The present invention is not necessarilylimited to this case. The plating film 64 fanned on the ground electrode31 may be removed and then the mark 60 may be formed on the non-platedportion 34 having no plating film 64. In this case, the mark 60 isformed after the tip 35 is welded and therefore background informationon the tip 35 and the welding can be recorded.

Although description is omitted in the embodiment described above, amark having an inverse relationship between the light module and thedark module may be provided on the ignition plug 10 as the mark 40, 60,or 70. In this case, the margins 43 and 63 of the marks 40 and 60 areparts of the first portions 41 and 61 (dark modules).

The embodiment described above is directed to the case in which the mark40 is provided on the metal terminal 20 having the wail portion 25 thatsurrounds the end face 24. The present invention is not necessarilylimited to this case. A metal terminal 20 without the wall portion 25may be used. Further, the mark 40 may be provided on the side face ofthe metal terminal 20 or the like instead of providing the mark 40 onthe end face 24 of the metal terminal 20.

Having described the invention, the following is claimed:
 1. An ignitionplug configured to ignite an air-fuel mixture in an internal combustionengine, the ignition plug comprising a mark that is formed of an oxidefilm generated on a surface of a metal member or is formed of the metalmember and the oxide film.
 2. The ignition plug according to claim 1,wherein the mark comprises: a first portion; and a second portion havinga reflectance higher than a reflectance of the first portion, andwherein a variation of distribution of the reflectance of the firstportion or a variation of distribution of the reflectance of the secondportion is smaller than a variation of distribution of a reflectance ofa portion on the surface of the metal member adjacent to a periphery ofthe mark or a variation of distribution of a reflectance of an oxidefilm adjacent to a periphery of the mark.
 3. The ignition plug accordingto claim 2, wherein the variation of the distribution of the reflectanceof the first portion and the variation of the distribution of thereflectance of the second portion are smaller than the variation of thedistribution of the reflectance of the portion on the surface of themetal member adjacent to the periphery of the mark or a variation ofdistribution of a reflectance of an oxide film adjacent to a peripheryof the mark.
 4. The ignition plug according to claim 1, furthercomprising: an insulator; a tubular metal shell that holds the insulatorinside; a ground electrode joined to the metal shell; and a tip that isjoined to the ground electrode and contains a noble metal, wherein theground electrode comprises a non-plated portion having no plating filmformed on a surface thereof, and wherein the mark is provided on thenon-plated portion of the ground electrode.
 5. The ignition plugaccording to claim 2, further comprising: an insulator; a tubular metalshell that holds the insulator inside; a ground electrode joined to themetal shell; and a tip that is joined to the ground electrode andcontains a noble metal, wherein the ground electrode comprises anon-plated portion having no plating film formed on a surface thereof,and wherein the mark is provided on the non-plated portion of the groundelectrode.
 6. The ignition plug according to claim 3, furthercomprising: an insulator; a tubular metal shell that holds the insulatorinside; a ground electrode joined to the metal shell; and. a tip that isjoined to the ground electrode and contains a noble metal, wherein theground electrode comprises a non-plated portion having no plating filmformed on a surface thereof, and wherein the mark is provided on thenon-plated portion of the ground electrode.
 7. The ignition plugaccording to claim 1, further comprising: an insulator extending in adirection of an axial line; and a metal terminal fixed to an end of theinsulator in the direction of the axial line, wherein the mark isprovided on an end face of the metal terminal in the direction of theaxial line.
 8. The ignition plug according to claim 2, furthercomprising: an insulator extending in a direction of an axial line; anda metal terminal fixed to an end of the insulator in the direction ofthe axial line, wherein the mark is provided on an end face of the metalterminal in the direction of the axial line.
 9. The ignition plugaccording to claim 3, further comprising: an insulator extending in adirection of an axial line; and a metal terminal fixed to an end of theinsulator in the direction of the axial line, wherein the mark isprovided on an end face of the metal terminal in the direction of theaxial line.
 10. The ignition plug according to claim 4, furthercomprising: an insulator extending in a direction of an axial line; anda metal terminal fixed to an end of the insulator in the direction ofthe axial line, wherein the mark is provided on an end face of the metalterminal in the direction of the axial line.
 11. The ignition plugaccording to claim 7, wherein the metal terminal comprises a wallportion that surrounds the end face.
 12. A method for manufacturing anignition plug configured to ignite an air-fuel mixture in an internalcombustion engine, the method comprising: a preparation step ofpreparing a metal member that constitutes a part of the ignition plug;and a marking step of forming a mark that is formed of an oxide film oris formed of the metal member and the oxide film by radiating laser beamonto a surface of the metal member.
 13. The method for manufacturing anignition plug according to claim 12, wherein the mark comprises: a firstportion; and a second portion having a reflectance higher than areflectance of the first portion, and wherein the marking step comprisesa first step of radiating the laser beam onto a portion on the surfaceof the metal member where the first portion is to be formed or a portionon the surface of the metal member where the second portion is to beformed.
 14. The method for manufacturing an ignition plug according toclaim 13, wherein the marking step further comprises, after the firststep, a second step of radiating the laser beam onto the portion wherethe first portion is to be formed but the laser beam is not radiated inthe first step or the portion where the second portion is to be formedbut the laser beam is not radiated in the first step.
 15. The method formanufacturing an ignition plug according to claim 13, wherein themarking step further comprises, before the first step, an underlayerforming step of forming an underlayer region by radiating the laser beamonto a portion where the mark is to be formed.
 16. The method formanufacturing an ignition plug according to claim 14, wherein themarking step further comprises, before the first step, an underlayerforming step of forming an underlayer region by radiating the laser beamonto a portion where the mark is to be formed.
 17. The method formanufacturing an ignition plug according to claim 13, wherein the firstportion is formed by promoting formation of the oxide film throughradiation of the laser beam, and wherein the second portion is formed byremoving the oxide film through radiation of the laser beam.
 18. Themethod for manufacturing an ignition plug according to claim 14, whereinthe first portion is formed by promoting formation of the oxide filmthrough radiation of the laser beam, and wherein the second portion isformed by removing the oxide film through radiation of the laser beam.19. The method for manufacturing an ignition plug according to claim 15,wherein the first portion is formed by promoting formation of the oxidefilm through radiation of the laser beam, and wherein the second portionis formed by removing the oxide film through radiation of the laserbeam.
 20. The method for manufacturing an ignition plug according toclaim 17, wherein the first portion is formed by promoting formation ofthe oxide film through radiation of the laser beam, and wherein thesecond portion is formed by removing the oxide film through radiation ofthe laser beam.